I recently came across focus@will, a site that claims:
Focus@will technology delivers various "Attention Amplifying" music channels scientifically designed to engage with your brain's limbic system. This soothes the easily distracted fight or flight mechanism increasing attention span and general focus.
And so far, I have found it quite useful. The Up Tempo channel and a set of good over the ear headphones really help me concentrate.
Really worth the money for a Pro account.